Determination of iso-electric point of silicon nitride by adhesion method

The iso-electric point (i.e.p.) of silicone nitride is an important parameter in the fine ceramics production technology. The determination of its value may present a problem, primarily because of surface reactivity of silicone nitride. For this purpose the adhesion method for i.e.p. determination was developed. In this paper the batch method was demonstrated. The kinetics of attachment of silicone nitride colloid particles on titanium collector beads was followed. The closed cylinders containing colloidal suspension of silicone nitride together with certain amount of titanium beads were rotated, and the particle number concentration of silicone nitride was measured as a function of time. From the experimental data the rate constants of attachment were determined by considering the reverse detachment process. The i.e.p. of titanium is at pH 2, so that titanium surface was negatively charged in the whole examined pH region. In the region of pH < 8 silicone nitride was positively charged which promoted the attachment. At pH > 8, both silicone nitride and titanium surface are negatively charged which significantly reduces the attachment rate. The theoretical rate constant in absence of interaction forces (k(diff)) was calculated by considering the convection diffusion of colloid particles towards the moving beads. The velocity of titanium beads, with respect to liquid suspension, was calculated, taking into account gravitation, bouyancy and friction forces. The pH at which the experimental rate constant coincides with the theoretical k(diff) value was taken as the iso-electric point. The results were confirmed by independent electrokinetic measurements. Preliminary experiments in non-aqueous medium indicated the applicability of adhesion method also to these systems. (C) 2000 Elsevier Science B.V. All rights reserved.